Permanent casting die with ceramic lining

ABSTRACT

In a permanent casting die with an at least local ceramic lining, the ceramic lining at least partially forms surface contours of a die cavity. The ceramic lining is applied in a form-fitting manner to a metallic core and the ceramic lining with the core is anchored firmly or movably in the die cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to Application No. 102 07989.7, filed in the Federal Republic of Germany on Feb. 25, 2002, whichis expressly incorporated herein in its entirety by reference thereto.

FIELD OF THE INVENTION

[0002] The present invention relates to a permanent casting die and to amethod for producing a ceramic lining for a permanent casting die.

BACKGROUND INFORMATION

[0003] Permanent casting dies, both in gravity casting and especially inpressure die casting, are exposed to very high thermal and mechanicalloads, which ultimately lead to steady wear. In series production, thisleads to continuous overhauling and renewal of the casting dies. Thisgives rise to very high costs, which in turn have a sustained effect oncomponent costs.

[0004] European Published Patent Application No. 0 065 996 describes ametallic casting mold which has ceramic elements as a lining. The liningcan be removed easily and is suitable especially for small individualcomponents. However, the lining in the configuration described cannot beused for complex large components subject to high loads and is notcapable of withstanding high mechanical stresses in continuousoperation.

[0005] It is an object of the present invention to configure a permanentcasting die such that wear may be reduced and that remachining cyclesmay be increased.

SUMMARY

[0006] The above and other beneficial objects of the present inventionare achieved by providing a permanent casting die and a method asdescribed herein.

[0007] In an example embodiment of a permanent casting die according tothe present invention, the permanent casting die has a ceramic liningwhich is firmly connected to a metallic core. For simplicity, theceramic lining with the core is referred to below as a composite. Thecomposite is inserted in a fixed or movable manner into a die cavity ofthe permanent casting die, the ceramic lining forming parts of thesurface contour of the die cavity. In certain cases, it is possible forpartial areas of the core to lie at the surface.

[0008] The effect of this arrangement is that the comparatively brittleceramic has a ductile support and, on the other hand, the ceramicsurface is extremely wear-resistant.

[0009] The service life of the composite may be increased even furtherif the coefficient of thermal expansion of the core and the lining isapproximately the same. The coefficient of expansion of the core and thelining may differ by no more than 4×10⁻⁶ K⁻¹, e.g., by no more than2.7×10⁻⁶ K⁻¹, absolute.

[0010] This may be achieved by pairing materials in various manners.Thus it is possible to use any industrial ceramic, e.g., aluminum oxide,aluminum nitride, silicon or titanium carbide, hard metals and siliconnitride, etc., for the ceramic lining. Ceramic which exhibits almost nowetting, if any, with respect to a casting metal, which may be analuminum alloy, may be suitable. This applies, e.g., to silicon nitrideand aluminum nitride.

[0011] All high-melting metals or metal alloys may be suitable as corematerials. A suitable pair of materials may be silicon nitride and amolybdenum alloy. The difference between the coefficients of expansionof these materials may be small. The coefficients of expansion may beadditionally aligned by alloying additives and/or additives to ceramicraw materials. It may be provided that the metal of the core has a highthermal conductivity to ensure as rapid as possible heat dissipation.This applies to molybdenum as a core material inasmuch as its thermalconductivity is about 140 W/mK. In this context, it may be provided thatthe core accounts for the highest proportion of the volume of thecomposite since the core may provide better heat dissipation than theceramic lining.

[0012] The ceramic lining may be only as thick as is required by thegeometrical conditions. In the case of a self-supporting ceramic lining,the wall thickness of the lining is between 2 mm and 50 mm, e.g.,between 3 mm and 15 mm. Ceramic linings applied in layers or bysintering have wall thicknesses of between 0.05 mm and 20 mm, e.g.,between 0.5 mm and 5 mm.

[0013] In an example embodiment of the present invention, the compositeconstitutes a largely cylindrical mandrel. In this example embodiment,the core may be cylindrical in shape, while the ceramic lining may bematched in a form-fitting manner to the cylindrical core.

[0014] Certain advantages of the present invention may be achieved whenthe permanent casting die is a pressure die-casting die (pressuredie-casting tool). The stresses on the permanent casting die are highestand the wear most severe in the pressure die-casting method (all castingmethods in which casting is carried out at increased pressure).

[0015] Cylinder/crankcases are components which require a particularlycomplex casting tool and are subject to high wear. The application ofthe present invention to a crankcase-casting tool may be advantageous,e.g., in its application to one or more mandrels reproducing thecylinder bores of the cylinder/crankcase. The mandrels may be subject towear. The mandrels are mounted in the casting tool such that they maymove and, after the casting of the crankcase, they move out of the toolfor the removal of the crankcase from the die. As they move out, themandrels are once again subjected to increased wear.

[0016] The form-fitting connection between the ceramic lining and thecore may be achieved through a large number of measures. An undulating(sinusoidal) core cross-section may be suitable, for example. Thesurface area of the core and hence the area of contact between thelining and the core may be increased by this refinement, resulting inbetter heat transfer. The enlargement of the areas of contact may beprovided for the removal of the surface heat.

[0017] For better matching of the ceramic lining and of the core, thecore may be provided with a cone. The ceramic lining has a mating cone,leading to a firm, form-fitting connection.

[0018] A thermally conductive paste, which is applied at least partiallyto the areas of contact, may help to promote heat dissipation from thesurface to the core. The thermally conductive paste, which may bemetal-based, e.g., zinc-based, titanium-based, etc., may provide theeffect of balancing out differences in tolerances. Thus, the ceramiclining and/or the core may be produced with relatively wide tolerances,reducing machining costs.

[0019] In an example embodiment of a method for the production of aceramic lining of a permanent casting die according to the presentinvention, for the form-fitting matching and production of the ceramiclining, a raw ceramic compound is applied to a metallic core. This maybe performed by the application of any technical skill, e.g., bypressing on, by slip application, by the application of a ceramic greenbody, by the application of preceramic polymers, etc.

[0020] The raw ceramic compound with the core is subsequently solidifiedto form a ceramic. This is usually accomplished by a sintering processor, in the case of polymers, in the form of carbonization. The ceramicis then finish-machined, thereby generating the surface of the ceramiclining.

[0021] In another example embodiment of a method for the production of aceramic lining on a core according to the present invention, a ceramiclayer is applied to the core, e.g., by a thermal spraying method. Inthis manner, very thin ceramic linings with wall thicknesses of between0.05 and 5 mm, e.g., between 0.1 mm and 1.5 mm, may be achieved.

[0022] Example embodiments of the present invention are explained ingreater detail with reference to the following figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 illustrates a mandrel including a ceramic lining and ametallic core.

[0024]FIG. 2 is a fragmentary view of a permanent casting die with aceramic lining and a core.

[0025]FIG. 3a is a cross-sectional view through a mandrel, the corehaving an undulating cross-section.

[0026]FIG. 3b is a cross-sectional view through a mandrel, the corehaving grooves.

DETAILED DESCRIPTION

[0027] The mandrel 2 illustrated in FIG. 1 is part of a pressuredie-casting tool, which is used to produce a cylinder/crankcase. Themandrel 2 forms the aperture of the subsequent cylinder bore. It ismounted on a slide, by which it may be withdrawn after a castingoperation. The mandrel 2 has a molybdenum core 6, which is connected ina form-fitting manner to a ceramic lining 4 made of silicon nitride.Both the core 6 and the ceramic lining 4 have a cone 22, by which aplay-free composite may be ensured. The contact surfaces 5 between thecore 6 and the ceramic lining 4 are coated with a zinc paste. The zincpaste serves for heat transmission from the ceramic 4 to the core 6. Thecoefficient of thermal expansion of the molybdenum is approximately5.6×10⁻⁶ K⁻¹, while the coefficient of thermal expansion of the siliconnitride is about 3×10⁻⁶ K⁻¹.

[0028] The mandrel 2 may optionally have cooling channels passingthrough it in the core 6. In the example embodiment illustrated in FIG.1, the mandrel 2 has a free core surface 3, which is used to form asurface contour. This free core surface may optionally be chosen withregard to the thermal and mechanical requirements in the respectivepermanent casting die. By virtue of the ceramic lining according to thepresent invention, the service life up to remachining of the mandrel maybe tripled compared with conventional steel-based mandrels.

[0029] In another example embodiment of the present inventionillustrated in FIG. 2, one edge 11 of a permanent casting die 8 isreinforced locally by a ceramic lining 9 according to the presentinvention. The ceramic 9 is connected in a formfitting manner to amolybdenum core 7. The composite is secured on the permanent casting die8 on the rear side by a screw fastener 10.

[0030] Examples of core cross-sections are illustrated using a mandrelcore (14, 18) as an example in FIGS. 3a and 3 b. The core 14 of themandrel 12 has an undulating cross-section, which enlarges the area ofcontact with the ceramic lining 13. This accelerates heat transfer fromthe core 14 to the ceramic 13. The same effect is achieved by themeasure illustrated in FIG. 3d, in which the core 18 has grooves 20,onto which the ceramic lining 17 is mounted. Both the grooves 20 and thewave shape of the core 14 serve to fix the ceramic 13, 17 on the core14, 18.

[0031] The production of the composite according to the presentinvention may be achieved in, e.g., three manners. On the one hand, thecore and the ceramic lining may be produced separately, finish-machinedand then joined. Joining may be accomplished in a conventional manner.This may be performed by vacuum soldering, for example, leading to goodheat transfer between the core and the lining, and any gaps may beeliminated.

[0032] The second variant includes applying the ceramic as a raw ceramiccompound to a prefabricated core. The ceramic is sintered on the core,it being possible for sintering to take place under pressure, e.g., byhot pressing. The complete composite is then finish-machined for thepurpose of matching in the permanent casting die.

[0033] By the second variant, it is possible to dispense with thefinish-machining of the contact surfaces. The formfitting connection maybe ensured by the sintering process. On the other hand, shrinkage of theraw ceramic compound occurs during the sintering process, and this maybe taken into account beforehand. In the case of various geometricvariants, gaps may thus occur between the core and the ceramic. This maybe minimized by hot pressing for example. The particular productionmethod for the composite according to the present invention may bedetermined from the geometric requirements of the composite.

[0034] A third variant includes a ceramic layer being applied to thecore. The layer may be applied by any conventional ceramic coatingmethod. Thermal spraying methods, such as plasma spraying, flamespraying, wire arc spraying, etc., may be provided, for example.

What is claimed is:
 1. A permanent casting die, comprising: a core; aceramic lining configured to at least partially provide surface contoursof a die cavity, the ceramic lining applied form-fittingly to the core,the ceramic lining and the core anchored one of firmly and movably inthe die cavity.
 2. The permanent casting die according to claim 1,wherein an expansion coefficient of the core and an expansioncoefficient of the ceramic lining differ by a maximum of 5×10⁻⁶ K⁻¹. 3.The permanent casting die according to claim 1, wherein a composition ofthe ceramic lining is based on silicon nitride and a composition of themetallic core is based on molybdenum.
 4. The permanent casting dieaccording to claim 1, wherein the core and the ceramic lining form amandrel.
 5. The permanent casting die according to claim 1, wherein thecasting die is arranged as a pressure die-casting die.
 6. The permanentcasting die according to claim 5, wherein the pressure die-casting dieis configured to produce a cylinder/crankcase.
 7. The permanent castingdie according to claim 1, wherein the core includes one of an undulatingand fluted surface, the ceramic lining anchored to the one of theundulating and fluted surface.
 8. The permanent casting die according toclaim 1, wherein the core is approximately cylindrical and includes acone, the ceramic lining including a negative of the core.
 9. Thepermanent casting die according to claim 1, further comprising athermally conductive paste arranged at an interface between the ceramiclining and the core.
 10. A method of producing a ceramic lining of apermanent casting die including a core and a ceramic lining configuredto at least partially provide surface contours of a die cavity, theceramic lining applied form-fittingly to the core, the ceramic liningand the core anchored one of firmly and movably in the die cavity,comprising: applying a raw ceramic compound to a metallic core;subjecting the raw ceramic compound and the core to heat treatment tosolidify the ceramic; forming a firm connection between the ceramic andthe core; and finish machining a surface of the connection.
 11. A methodof producing a ceramic lining of a permanent casting die including acore and a ceramic lining configured to at least partially providesurface contours of a die cavity, the ceramic lining appliedform-fittingly to the core, the ceramic lining and the core anchored oneof firmly and movably in the die cavity, comprising: applying theceramic lining to the core by a ceramic coating method.